(a) Technical Field
The present invention relates to a method for controlling the temperature of a fuel cell system. More particularly, it relates to a method for controlling a coolant pump and a cooling fan in a fuel cell system, which can reduce the power consumption of the coolant pump and the cooling fan by optimizing their operations, prevent the occurrence of flooding and dry-out, and improve the performance of the fuel cell system and the water removal efficiency.
(b) Background Art
Recently, automobile manufactures have begun to incorporate fuel cell systems in to automobiles to provide an environmentally friendly hydrogen fuel cell vehicle as an alternative power generation source for consumers. Typically, a fuel cell system includes a fuel cell stack for generating electricity by an electrochemical reaction between reactant gases, a fuel processing system (FPS) for supplying hydrogen as a fuel to the fuel cell stack, an air processing system (APS) for supplying oxygen containing air as an oxidant required for the electrochemical reaction in the fuel cell stack, and a thermal management system (TMS) for removing reaction heat from the fuel cell stack to the outside of the fuel cell system, controlling operation temperature of the fuel cell stack, and performing water management function.
The fuel cell stack generates electricity via an electrochemical reaction between hydrogen and oxygen containing air as the reactant gases, and discharges heat and water as reaction by-products. Therefore, it is necessary to provide a system for maintaining the temperature of the fuel cell stack at an optimal level in the fuel cell system in order to provide for the most efficient processing.
In a typical fuel cell system for a vehicle, a method of circulating water through bipolar plate channels in the fuel cell stack is used for controlling the temperature of the fuel cell stack (e.g., a temperature control system or cooling system).
An example of the temperature control system for the fuel cell system is shown in FIG. 1. As shown in the figure, the temperature control system for the fuel cell system has a radiator 20 and a cooling fan 21 provided to transfer heat from coolant to the outside, a coolant line 31 provided between a fuel cell stack 10 and the radiator 20 such that the coolant is circulated therethrough, a bypass line 32 and a three-way valve 40 provided to bypass the coolant line 31 such that the coolant does not pass through the radiator 20, and a coolant pump 50 provided to pump and transport the coolant through the coolant line 31.
The bypass line 32 is a coolant line branched from the coolant line 31 on the upstream and downstream sides of the radiator 20 to bypass the coolant line 31 such that the coolant does not pass through the radiator 20. The three-way valve 40 functions to selectively control the flow of the coolant between the main line, which is connected to the radiator 20, and the bypass line 32, which is not connected to the radiator 20.
Furthermore, in a fuel cell system hydrogen and air are received from the outside and electricity and water are generated in the fuel cell stack. The water as a by-product of the electrochemical reaction may be in the form of vapor, saturated liquid, ice., etc. according to real-time conditions such as temperature, pressure, etc., and thus the transfer characteristics of water is changed over time. Moreover, this water affects the transfer characteristics of gas and electrons passing through the bipolar plate channels, a gas diffusion layer, a catalyst layer, and an electrolyte membrane. Accordingly, a flooding phenomenon due to an excessive amount of water and a dry-out phenomenon due to a shortage of water coexist in the fuel cell system.
To solve these problems, the temperature control of the fuel cell stack is the core concern, and it is necessary to develop control logic for solving the flooding and dry-out phenomena, while minimizing the power consumption by optimizing the operation of the coolant pump and the cooling fan.
U.S. Pat. No. 6,087,028 describes a method for controlling a coolant pump and a cooling fan to maintain a difference between a coolant inlet temperature and a coolant outlet temperature within a specific temperature range using a temperature distribution detection unit and a load state detection unit.
Here, if the difference between the coolant inlet temperature and the coolant outlet temperature is greater than a predetermined value, the operating voltage of the coolant pump is increased, and if the flow rate of external air detected by an intake air state detection unit is greater than a predetermined value, the flow of the external air into a radiator is restricted (the cooling fan is rotated in a reverse direction).
However, the above method can be implemented when the coolant pump and the cooling fan are operating under severe conditions, which is disadvantageous in terms of fuel efficiency.
Moreover, when the difference between the coolant inlet temperature and the coolant outlet temperature increases during low-temperature operation, it is advantageous in terms of water removal, and thus it is undesirable to restrict the difference between the coolant inlet temperature and the coolant outlet temperature with excessive power consumption in this situation.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.